A water-absorbing resin (SAP/Super Absorbent Polymer) is a water-swellable, water-insoluble polymer gelling agent, and is often used for absorbents such as sanitary materials including disposable diapers and sanitary napkins, agricultural and horticultural water retaining agents, and industrial water blocking materials.
Such water-absorbing resins are produced using a variety of monomers or hydrophilic polymers as raw materials. Of these, a polyacrylic acid (salt)-based water-absorbing resin produced using acrylic acid and/or salt thereof as monomer is most commonly produced industrially because of its high water absorption performance.
The water-absorbing resin is produced through a polymerization step, a gel-crushing step, and a drying step, and optionally, through a pulverizing step, a classification step, a surface-crosslinking step, and/or the like. As disposable diapers, which are major applications of the water-absorbing resin, grow in performance, the water-absorbing resin is also required to have various functions (physical properties). Specific examples of such physical properties include not only high fluid retention capacity but also high gel strength, low water-soluble content, high water absorption speed, high fluid retention capacity under pressure, high liquid permeability, small particle size distribution, urine resistance, antibacterial property, impact resistance (damage resistance), powder fluidity, deodorizing property, coloration resistance (whiteness), and low dust generation.
Of the physical properties listed above, liquid permeability and water absorption speed are important as basic physical properties of the water-absorbing resin, and many improvement techniques have been proposed. However, since high liquid permeability and high water absorption speed are the physical properties in a trade-off relationship, there has been a demand for a technique to achieve both high liquid permeability and high water absorption speed.
One such technique to achieve both high liquid permeability and high water absorption speed has been proposed, by which a crosslinked hydrogel polymer obtained through a polymerization step is crushed in specific gel-crushing conditions to obtain a water-absorbing resin having an excellent liquid permeability and an excellent water absorption speed (refer to Patent Literature 1).
It is noted that techniques to improve physical properties of a water-absorbing resin have been proposed, by which various additives are added in a polymerization step, a gel-crushing step, a granulation step, and/or the like (refer to Patent Literatures 2 to 9).
Furthermore, in regard to measurement of swollen water-absorbing resin particles (i.e., gel particles), methods of measuring the weight average particle diameter (D50) and the particle size distribution of a particulate hydrogel resulting from gel-crushing are disclosed (Patent Literature 1, Patent Literatures 10 and 11).